I would use a CAN Interface. Physicaly it is based on RS485, but there are some chips out there (see philips) that do all you need.
These chips are designed for industrial inviroment and so they should be save enough.
CAN is ment to do what you want to do. each device can be the master and the slave. so all devices can initiate a transfer to any other device on the net.

the realisation is much more easier than this sounds. just 2 little chips and some stuff like Rs and Cs. Most of the protocol is done in these chips, so you just need some smal software to control them.

there are also several cpus out there with CAN build in. Dallas DS80C390, Siemens SAB-C515C, ....

Michael